Body organ stimulator with interference rejector

ABSTRACT

A pace pulse generator connects to an organ which is to be electrically stimulated. Natural and artificial stimulating signals on the body are detected and used to control the pace pulse generator. An interference rejector distinguishes between detected natural body signals and extraneous electrical interference. The rejector is also adaptable for control of atrial and ventricular tachycardia and arrhythmia in connection with heart stimulators.

United States Patent Bowers [15] 3,677,251 [451 July 18, 1972 [s41 BODYORGAN STIMULATOR WITH INTERFERENCE REJECTOR 3,431,912 3/1969 KellerJr...128/4l9P Primary Examiner-William E. Kamm Attorney-Jon Carl Gealou,Frank L. Neuhauser, Oscar B. Waddell, Joseph B. Forman and Arthur V.Puccini ABSTRACT A pace pulse generator connects to an organ which is tobe electrically stimulated. Natural and artificial stimulating m 128/419P signals on the body are detected and used to control the pace '3- 1/34pulse generator. An interference rejector distinguishes [58] Field ofSearch 128/419-423 between detected natural body signals and extraneouselectri cal interference. The rejector is also adaptable for control of[56] References Cited. atrial and ventricular tachycardia and arrhythmiain connec- UNITED STATES PATENTS $1011 with heart stimulators.

3,528,428 9/1970 Berkevits ..l28/4l9 P 1 Claim, 1 Drawlngligure PeE-AMPe20 1*2/ g ea 25 J I l T F B Patented July 18, 1972 Qmuk WW BODY ORGANSTIMULATOR WITH INTERFERENCE REJECT OR BACKGROUND OF THE INVENTIONElectric pulse generators have been used internally and externally ofthe body for stimulating a body organ to function in the absence ofnatural electric or nerve impulses. Stimulation of the bladder, ureterand the atrium and ventricles of the heart, are common examples. Whencardiac stimulators are used in subjects which have certain types ofdefects in the conduction system of their hearts, it is important to beable to distinguish between natural and control stimulating potentialsand artificial stimulating pulses so that if the former is present thelatter will not compete with it and cause a functional irregularity inthe heart.

In order to minimize possibility of competition, to let the heartfunction in response to its natural electric potentials whenever it can,and to conserve battery power, a standbytype of Pacemaker has beendeveloped. This device is otherwise known as a standby or demand heartpacer. The pacer has a pulse generator which connects to the heart andstimulates it with a substituted artificial pulse for every naturalstimulating pulse that is absent. When a natural beat occurs, the nextstimulating pulse that would ordinarily be furnished by the pulsegenerator, is inhibited. An improved standby pacer which permits theheart to operate over a physiologically acceptable range of rates beforeany artificial stimulus is applied, is described in the co-pendingapplication of the instant inventor, filed Mar. 10, 1970, Ser. No.18,051. This application is assigned to the assignee of the instantapplication.

Standby and demand pacers, the older type of fixed rate pacers, andother organ stimulators too are subject to their normal operating modebeing disrupted by interferring electric signals which are generated bymuscle potentials and the like in the body or which appear in the bodyas a result of their being radiated from an extraneous source. A commontype of interference is that which is radiated by electric shavers,electric drills and auto ignition. This type of interference and eveninterference coming from ordinary power lines has been known toadversely affect prior types of stimulators by causing them to turn offwhen they should be on or vice-versa, or to revert to some other type ofundesirable operating mode which defeats their intended purposes.

Another problem with prior stimulators is that on some occasions anorgan could function at an abnormally high rate, such as in tachycardiaof the heart and arrhythmic disturbances, and the stimulator would losecontrol over the organ. Stimulators of the standby-type would sense thehigh activity and turn off and stay off or would turn on and stay on. Ithas been discovered, however, that with some organs, if artificialstimulating pulses are injected at an appropriate time, natural rhythmof the organ will be restored and control by the stimulator may berecaptured.

SUMMARY OF THE INVENTION An object of the present invention is toovercome some of the above disadvantages and to provide new stimulatorfunctions by means of a novel interference rejector.

Another object is to provide an interference rejector which candistinguish desirable pulses of one repetition rate range from pulses ofanother repetition rate range which are as sociated with interference sothat suitable control signals may be derived to control the operatingmode of the pace pulse generator or other component of a stimulator.

A more specific object of this invention is to provide an interferencerejector which, in addition to it being able to distinguish betweennaturally produced pulses and pulses due to interference, is capable ofdistinguishing between normal and abnormal functional rates of the organresulting from response to its own electric impulses. This object isexemplified in a away or turned off or functioned randomly or exhibitedsomeother malfunction in the presence of interference. Interference maybe defined as any unwanted signals which disrupt intended operation ofthe pace pulse generator. For test purposes, control signals may beinjected into the detector to produce system response needed foranalyzing system performance according to the invention. Control signalsmay be of proper intensity to provide control.

Although the concepts for the interference rejector herein proposed aregenerally applicable to stimulators, the invention will be describedprimarily as it relates to a standby heart pacer. Those skilled in theart will appreciate how to apply the principles of the invention todifierent kinds of heart pacers and other stimulators as well.

In general terms, the illustrative heart pacer comprises a pace pulsegenerator which controls a stimulating pulse generator whose outputterminals are connectable to the heart to stimulate it in the absence ofnatural heart impulses. There is also a detector system for detectingboth body generated, control, and artificial stimulating signals withinthe body. The detector delivers corresponding output pulses to theinterference rejector. The interference rejector receives pulses whichare properly processed and gated along with pulses that result frominterference. If the combined input pulse rate to the rejector is toohigh or outside of limits, the rejector has one output state and if thepulses are at a rate that is within the predetermined range, therejector reverts to another output state. These alternate states areemployed to exercise a control function over the pace pulse generator soit will inject stimulating pulses to recapture control if the heart isin tachycardia, or if there is significant interference, it willstimulate the heart at a known rate rather than allow the heart to missbeats, thereby failing-safe if there is significant interference.

A more detailed description of an illustrative embodiment of theinvention will now be set forth in reference to the drawmg.

v DESCRIPTION OF THE DRAWING The drawing shows a circuit diagram of astandby heart pacer with the new interference rejector and tachycardiaand arrhythmic control incorporated therein.

DESCRIPTION OF A PREFERRED EMBODIMENT At the far right of the drawing anorgan to be stimulated, such as a heart 10, is depicted. The standbypacer output terminals l2, 13 may be connected to the heart by leads 1 Iwhich may attach to the myocardium or which may be an in travenousconductive catheter for stimulating interiorly of the heart. Leads 11may serve the dual purpose of delivering artificial pacing pulses to theheart and detecting the presence of natural heart potentials althoughseparate leads could be used for these purposes.

Except for leads 11, the other circuit elements shown are usuallyencapsulated in solid epoxy resin and coated with a body compatiblesilicone moisture barrier so that the device may be implanted in thebody. The device may also be situated outside of the body in which casean intravenous conductive catheter leading to the heart may providestimulating pulses and also detect any signals on the heart.

The pacer is powered by a six-cell battery 14 which has an initialterminal voltage of about 8 volts in this case.

At the left of the drawing, the first stage of the pacer is shown inblock form and includes a preamplifier and twin-T filter identified bythe reference numeral 18. The frequency filter 18 is part of a detectorsystem. It should be noted that filter 18 is designed to be selective onthe basis of signal frequency content, not signal repetition rate as isthe case with the new interference rejector. Detected natural heartsignals and artificial heart stimulating pulses are introduced to thefilter by way of conductor 19 which connects to the heart by means of anoutput terminal 12. The pass-band of frequency filter 18 is preferablybetween 20 and 40 Hz. When a natural or artificial heart signal havingselected frequency components is received, filter 18 is shocked andproduces a ringing output signal of the form marked 20 on the outputside of the filter. Sometimes the polarity of the heart signal reversesbut the alternate ringing signal 20 is produced by the filter regardlessof whether the incoming signal is positive or negative. Interferencesignals'inherent in the body and derived from outside sources are alsodetected sometimes and even though they may produce ringing signals orappear elsewhere, their effect is vitiated by the interference rejectorwhich will be described hereafter;

The ringing signal 20 that corresponds with detected body signals to thefilter is a-c coupled through a capacitor 21 to a gated thresholdtrigger 22 which is shown in block form.

The input circuit to trigger 22 includes a bias voltage dividercomprising resistors 23 and 24. Trigger 22 is normally biased off but itwill turn on and emit an output pulse whenever it integrates thepositive swings of a ringing pulse 20. Trigger 22 may be adapted toproduce output pulses 25 for every incoming pulse 20 or it may beselectively inhibited so that only pulses occurring under certaincircumstances, such as of amplitude and duration, are transmitted.Inhibition is accomplished in this example by rendering a transistor 61conductive at the proper moment, in which case, the bias point betweenresistors 23 and 24 is clamped to ground and the trigger 22 isinsensitive to incoming signals. An occasion for inhibiting would bewhen it is desired to prevent artificial stimulating pulses which arealso detected on the heart and the body from producing an output pulse25 from the trigger. Normally, there will be a pulse 25 coincident withevery natural heart heat which means that pulses 25 will normally occurwithin physiological rate limits. Higher rate pulses are usuallyassociated with interference or with the heart undergoing tachycardia orarrhythmia.

Pulses 25 appear at the top of a resistor 26 from which they are appliedto the input of a novel interference rejector which is enclosed in adash-dot rectangle and labelled 27. The interference rejector willhereafter be called a rejector for brevity.

In one embodiment, the rejector is adapted to reject repetitive signalshaving an interpulse period which corresponds to a rate greater thanl20pulses per minute. In many cases, this rate coincides with the desiredphysiological limits. Signals with a shorter period or greater rate mayresult from existence of interference caused by electric shavers,electric drills, auto ignitions and the like. A shorter period betweenany two consecutive signals may also occur if the heart is arrhythmic orin tachycardia.

Although for ordinary interference rejection, such as used in thestandby pacer, stable supply voltage is not imperative, such stablevoltage is important-and required if the rejector is adapted fortachycardia and arrhythmic control. For this reason, the trigger pointof the threshold trigger 22 is made very stable by use of a zener diode39 having a current limiting resistor 38 in series with it. Use of thezener diode enables stable operation, although the battery 14 issomewhat depleted, as long as the battery voltage has not dropped belowzener voltage. In pacers that are modified for tachycardia andarrhythmic control, the pulse width from the trigger to the inputcapacitor 28 of the rejector must always be long enough to fully chargecapacitor 28 to a predetermined fixed level. This accounts, in part, forthe need of a very stable supply voltage.

The rejector includes capacitor 28 and a series charging pathfor itcomprising a low parallel resistance value resistors 29 and 36 in serieswith a diode 30 which is forward-biased when the capacitor charges.Thus, pulses 25 will charge capacitor 28 to a certain level with a shortcharging time constant. Between pulses, capacitor 28. is dischargedslowly through a circuit which includes resistor 26, a ground path, ahigh value resistor and parallel resistors 29, 36. Diode 30 isreverse-biased during discharge of capacitor 28. Hence, the capacitorcharges rapidly and discharges slowly.

If pulses 25 are delivered to coupling capacitor 28 at a relatively lowrate, the interpulse period will have been sufficiently long to permitthe capacitor to discharge appreciably. An ensuing pulse will thendeliver charge to the capacitor and this charge will flow through thelow resistance parallel combination of resistors 29 and 36 and diode 30.This will produce a voltage at the top of resistor 29. If the voltage isgreat enough, current will also flow through zener diode 31 and a pulse32 will appear on the anode side of the zener diode and at the base oftransistor 33.

If pulses due to detected body signal, control signal or interferencesignals are delivered to coupling capacitor 28 at a relatively highrate, there will be insufi'icient interpulse time for the capacitor 28to discharge appreciably. Consequently, high rate pulses will encountera coupling capacitor 28 that is more fully charged in which case littleor no initial current will flow through resistors 29 and 36 in serieswith diode 30 and the voltage produced on top of resistor 29 will berelatively small. Ifthat voltage is below the zener voltage of diode 31no pulse will be delivered through it. It is evident, therefore, thatfor high rate pulses the output terminal of the voltage sensitive zenerdiode will be in one state and for low rate pulses, it will have anotherstate.

It should be evident that the interference rejector can be made torespond to different pulse repetition rates by changing the values ofcoupling capacitor 28, resistor 25 or the voltage level of zener diode31. In addition, different amplitude pulses 25 will alter the rejectorsselection of various pulse repetition rates.

If zener diode 39 is omitted or if battery 14 voltage falls below thiszener voltage as the battery depletes, then the output pulses 25 fromtrigger 22 will be reduced proportionally in amplitude as batteryvoltage changes, thereby affecting the voltage at the top of resistor29, which controls the conduction of zener diode 31. It is apparent thata reduced pulse 35 amplitude below the zener 31 voltage will produce nooutput pulse 32. This results in the pace pulse generator operating inits fixed rate mode as will appear more clearly later.

Other voltage sensitive devices may, of course, be substituted for zenerdiode 31. Those skilled in the art will appreciate that a Schmitttrigger circuit or a saturating operational amplifier may be substitutedfor zener diode 31 to obtain a change of state in response to signalswhich are coupled through capacitor 28.

As stated earlier, there are several kinds of electric signals which maybe detected on the body. Atrial and ventricular signals associated withnatural stimulation of the heart are examples of rather high potentialsignals. There are other bioelectric potentials associated with bodychemistry, muscle action, nerve impulses and the like. Some of these maybe considered interference signals insofar as stimulation of aparticular organ is concerned. There are also signals due to extraneouselectrical interference that is picked up by the body or radiated to itfrom an outside source such as a sparking electrical appliance. Inaddition, signals for stimulating or controlling an organ, such aspulses from a heart pace pulse generator, may also be detected in or onthe body.

Assume, for example, that a tolerable heart rate range for a subjectbeing stimulated with a standby heart pacer is from 60 to beats perminute. A desirable and expected rate for a subject who is undergoingrelatively low physical activity would be about 68 to 75 beats perminute. A healthy heart that is beating under the influence of its ownconduction system would produce a corresponding number of detectablenatural electric impulses. At the same time, the body may be picking upextraneous electrical interference which may have many high frequencycomponents, but has the effect of relatively high rate discrete pulsesappearing on the body. In the case of a standby pacer, artificial pulsesmay also be applied to the body to stimulate the heart, usually at arate of about 71 beats per minute. Of course, a standby pacer isindicated in cases where the heart periodically responds to one or moreof its natural stimuli, in which case, fewer artificial stimulatingpulses are injected to maintain the heart at a minimum beating rate.

In the illustrated heart pacer, the interference rejector is used toprovide a control signal for turning on a heart pace pulse generator ifa natural heart beat is missed or delayed for a predetermined time andto also turn it on if there is overwhelming interference or if the heartexceeds some predetermined rate, such as, 120 beats per minute, which isthe case if it is in tachycardia or undergoing disturbances. In theinstant device, the pulse generator will also turn on if the batteryvoltage declines by a certain amount as was mentioned earlier.

The effect of artificial stimulating pulses may be eliminated from theinterference rejector 27 by inhibiting gated threshold trigger 22through the agency of clamping transistor 61 whenever an artificialpacing pulse is detected. Other signals, of course, are detected at therate at which they are produced and they cause pulses such as 25 to beproduced at a corresponding rate. Generally, the subject will not beexposed to extraneous electric interference, in which case, puls'es 25corresponding with the production and detection of natural heart stimuliwill be coupled to capacitor 28 and cause the output of the interferencerejector to be in one state. When high rate pulses occur as they do inthe presence of high rate interference, coupling capacitor 28 will notaccept more charge because of the short discharge period between pulsesand zener diode 31 will not be rendered conductive, in which case, theoutput of the interference rejector 27 will be in another state. It isthus evident that detected signals coming in at a desirable rate can bedistinguished from higher rate interference signals.

The interference rejector may be used to control tachycardia or otherarrhythmias by setting the values of resistors 35, 36 so that high ratesignals also associated with atrial and ventricular tachycardia orarrhythmia are detected and treated as high frequency interference. Ifthe discharge rate of capacitor 28 is slow by reason of resistor 35having a high value, then only relatively low rate pulses will becoupled through capacitor 28. If the discharge rate is madecomparatively faster, then higher rate pulses may be coupled throughcapacitor 28 and the voltage sensitive zener diode 31. Hence, byproperly choosing resistor 35, in particular, the rejector may be causedto assume one output state corresponding with pulses in an acceptableheart rate range and to assume another output state when the heart is inarrhythmia or tachycardia even for the period of two beats. This isaccomplished by controlling the pace pulse generator which is generallydesignated 51 and will now be described.

Pace pulse generator 51 has a timing network including series connectedresistors 57, 56, and 55 and a timing capacitor 54 all which areconnected between positive line and ground. Capacitor 54 charges andwill cause the pulse generator to turn on when it reaches a certainvoltage level unless it is prematurely discharged in whole or in part.There is a voltage divider including resistors 48, 49 for controllingthe bias on the pace pulse generator. Point A in the divider has aspecific positive value with respect to ground. A capacitor 50inparallel with resistor 49 stabilizes the d-c voltage at point A. Whentiming capacitor 54 charges to above a certain value, it causes atransistor 52 to become forward-biased through its emitterbase circuitand a resistor 58 leading to point A. As the timing capacitor 54charges, its voltage is increasing toward the point where transistor 52will be forward-biased for producing a pace pulse. Such a pace pulsewill be produced unless capacitor 54 is prematurely discharged asmentioned above. When capacitor 54 is charged sufficiently, it willapply the necessary current to forward-bias the emitter-to-base circuitof transistor 52 thru a path which includes resistor 58 and it alsorenders the emitter-to-collector circuit of transistor 52 conductive andcauses a pulse 59 to appear on the collector. The pulse duration isabout two milliseconds which is desirable for stimulating the heart. Thelatching voltage keeping this multivibrator circuit in conductionappears across resistor 58 and is additive to the bias voltage acrossresistor 48. The other transistor 53 in the pace pulse generator alsoconducts when an artificial pacing pulse is delivered and the potentialappearing on its emitter is applied to a resistor 92 and a conductor 60through the base of the gating transistor 61 at the far left of thedrawing. A filter network including parallel capacitor and a resistor 91prevents the emitter of transistor 53 from floating above groundpotential while providing emitter impedance during the conductive stateof transistor 53. The purpose of applying a signal to transistor 61 isto render it conductive and thereby inhibit the threshold trigger 22 sothat it does not sense or respond to artificialpacing signals. Whentransistor 61 conducts, the top of resistor 24 is placed at groundpotential and the bias is removed from threshold trigger 22 so itproduces no output pulse 25.

Output pulses 59 appearing on the collector of transistor 52 produce asignal voltage across a resistor 62 in the pacing pulse generator 51.This signal voltage is transmitted through a resistor 63 to the base ofa transistor 64. Transistor 64 has a current limiting resistor 65 in itsemitter circuit and a high value collector resistor 66. Between pulsesand between conduction intervals of transistor 64, a heart couplingcapacitor 67 slowly charges through high resistance 66. The slow chargerate is insufiicient to stimulate the heart. However, when a pulse isdelivered to transistor 64, and it becomes conductive, capacitor 67 isdischarged in a little more than two milliseconds and this results insufficient current flow to stimulate one heart beat. This process isrepeated as long as the pacing pulse generator is operating and theheart beats at the intrinsic rate of the pulse generator.

It should be noted that neither the output circuit including transistor64 and coupling capacitor 67 nor the pacing pulse generator consumeanything more than leakage current when they are in their standby state.

Timing capacitor 54 in the pace pulse generator is dischargedprematurely so it produces no pacing pulse in response to appropriatecontrol signals from the interference rejector which is equivalent tosaying that this happens when the output of the interference rejector isin an appropriate state.

Pulses which are transmitted through zener diode 31 in the interferencerejector are applied to the base of a transistor 33, forward-biasing itand rendering the transistor conductive. The base-emitter circuit isparallel with a filter capacitor 34. The collector of transistor 33connects to positive line through a collector-resistor 37. The collectorof transistor 33 is also connected to the collector of a paralleledtransistor 82 which becomes forward-biased and conductive when itsbase-emitter circuit which is in series with the emitter of transistor33 becomes conductive. The base-emitter circuit of transistor 82includes a filter network comprising capacitor 84 and resistor 83. Thecollector of transistor 82 is connected to timing capacitor 54 through aresistor 81 and a diode 80. When transistors 33 and 82 areforward-biased as a result of receiving a control pulse 32, thesetransistors become conductive and jointly discharge timing capacitor 54prematurely. That is, the timing capacitor is discharged before itreaches a sufficient voltage level to forward-bias transistor 52 in thepace pulse generator and turn it on.

It should be evident, therefore, that if the heart has produced anatural stimulus, such signal will be detected and transmitted throughthe interference rejector, causing timing capacitor 54 to be dischargedso that no artificial pulse will be applied to the heart. If, on theother hand, no natural stimulus occurs for a predetermined period oftime, such as one-sixtieth of a minute, timing capacitor 54 will reach avoltage level which will cause the pacing pulse generator to be turnedon for at least one beat so that the heart will be stimulatedartificially at the end of the one-sixtieth of a minute period. If thenext one or more natural stimuli are not detected, the pace pulsegenerator will continue stimulating at a higher rate such as everyone-seventieth of a minute because timing capacitor 54 will reach itsforward-biasing level earlier when it is not 54 in the rate generatorwill reach the level where it can turn on the pace pulse generator. Itis evident, therefore, that in the presence of interference, the pacepulse generator will turn on or fail-safe and provide stimulating pulsesto the heart to assure that it will beat in case natural stimuli do notoccur during the period of interference.

The interference rejector 27 may also be used to control arrhythmia ortachycardia of the heart. Under these heart conditions signals aredetected on the body which collectively look like high rate interferencein which case they will not get through coupling capacitor 28 in theinterference rejector. It

follows that zener diode 31 will not be conductive nor will a signal beproduced that would cause premature discharge of timing capacitor 54,in'which case, an artificial pulse is applied to the heart. The one ormore ensuing injected artificial stimulating pulses are often sufficientto depolarize the heart cells simultaneously and cause the heart topause after which natural rhythm can be restored.

Those skilled in the art will appreciate that the new interferencerejector may be used in other body organ stimulators as well as inimplantable and external standby heart pacers. It only necessary todetect the presence of body signals and supply them to the interferencerejector which will distinguish those that are in an acceptablerepetition rate range from those that are not, and will producealternate control signals that can be used to turn on or turn off thestimulator or provide some other control function as desired. It shouldalso be evident that the interference rejector can be designed tooperate inversely, that is, to control with the detected pulses having ahigh repetition rate instead of a low rate.

Another important application is for analyzing the function of implantedstimulators which have the new interference rejector incorporated. Thepacer described herein, for example, can be shifted to its fixed ratemode by intentional application of externally applied control signals.Some pacers sense atrial signals and produce a delayed ventricular pulseso the two heart chambers function in there natural sequence. Theinterference rejector can be adapted so that pulses may be injected at aproper rate to produce various systems responses and the function ofdifferent parts of the system can be individually determined.

Although an embodiment of the new'interference rejector has beendescribed in considerable detail in connection with a standby heartpacer, such description is intended to be illustrative rather thanlimiting for the rejector may be variously embodied and may be used witha variety of stimulators. According y the scope of the invention is tobe determined exclusively by interpreting the claims which follow.

I claim:

l. A body organ stimulator comprising:

a. a pulse generator that is adapted to be connected to the body andthat has a timing network which effects stimulating signals with apredetermined period,

b. detector means for detecting body signals comprised of naturalsignals, or artificial pace signals, and combinations thereof, and forproducing corresponding detector output signals,

c. an interference signal rejector comprising:

i. a resistor network comprising first and second circuits,

ii. a coupling capacitor connected between said detector means and saidresistor network to receive detector output signals directly from saiddetector means corresponding) with the detected boddy signals and saidcapacitor emg charged to a pre etermlned voltage level at one ratethrough said first circuit of said resistor network and to be dischargedat a different rate through said second circuit of said resistornetwork,

iii. a signal level sensing device connected at the junction of saidcoupling capacitor and said resistor network for sensing a voltage onsaid resistor network which corresponds to the difference between thedetector output signal and the voltage on said coupling capacitor, thevoltage on said capacitor depending on the repetition rate of thedetector output signals which charge it and the voltage on the resistornetwork during occurrence of a detector output signal depending on thelevel to which the capacitor is discharged between such signals,

iv. said signal level sensing device having different output statesdepending on the magnitude of the difference between the detector outputsignal and the voltage on said coupling capacitor,

said signal level sensing device having a first output state when thedetector output signals have a repetition rate within certain limits,and having a second output state when the detector output signals have arepetition rate outside the certain limits,

d. switching means responsive to one state of said signal level sensingdevice by being nonconductive and to another state by being conductive,and

e. a capacitor connected to said switching means and said pulsegenerator, operation of said pulse generator depending on the voltage onsaid capacitor resulting from conduction or nonconduction of saidswitching means and the state of said level sensing device.

nun,

1. A body organ stimulator comprising: a. a pulse generator that isadapted to be connected to the body and that has a timing network whicheffects stimulating signals with a predetermined period, b. detectormeans for detecting body signals comprised of natural signals, orartificial pace signals, and combinations thereof, and for producingcorresponding detector output signals, c. an interference signalrejector comprising: i. a resistor network comprising first and secondcircuits, ii. a coupling capacitor connected between said detector meansand said resistor network to receive detector output signals directlyfrom said detector means corresponding with the detected body signalsand said capacitor being charged to a predetermined voltage level at onerate through said first circuit of said resistor network and to bedischarged at a different rate through said second circuit of saidresistor network, iii. a signal level sensing device connected at thejunction of said coupling capacitor and said resistor network forsensing a voltage on said resistor network which corresponds to thedifference between the detector output signal and the voltage on saidcoupling capacitor, the voltage on said capacitor depending on therepetition rate of the detector output signals which charge it and thevoltage on the resistor network during occurrence of a detector outputsignal depending on the level to which the capacitor is dischargedbetween such signals, iv. said signal level sensing device havingdifferent output states depending on the magnitude of the differencebetween the detector output signal and the voltage on said couplingcapacitor, said signal level sensing device having a first output statewhen the detector output signals have a repetition rate within certainlimits, and having a second output state when the detector outputsignals have a repetition rate outside the certain limits, d. switchingmeans responsive to one state of said signal level sensing device bybeing nonconductive and to another state by being conductive, and e. acapacitor connected to said switching meAns and said pulse generator,operation of said pulse generator depending on the voltage on saidcapacitor resulting from conduction or nonconduction of said switchingmeans and the state of said level sensing device.